1. Field of the Invention
The present invention relates to a structure monitor system for measuring a physical quantity such as temperature, distortion or the like of a structure using an optical fiber sensor.
2. Description of the Related Art
In recent years, there have been developed structure monitor systems for measuring and monitoring a change in a physical quantity of a structure such as a tunnel, a bridge or a building using a distribution-type optical fiber sensor for measuring a distribution of the physical quantity such as temperature and distortion making the most use of properties of an optical fiber.
For example, Japanese Unexamined Patent Publication No. H11-287650 discloses a structure monitor system for detecting the deformation of a tunnel based on information from a distribution-type optical fiber sensor laid on the inner circumferential surface of the tunnel in a meandering manner along the longitudinal direction of the tunnel.
Specifically, this structure monitor system is constructed such that the looped optical fiber sensor, which is looped a specified number of times per unit looped section length, is laid along the longitudinal axis of the tunnel while reciprocating along the circumferential direction of the tunnel, and a distortion distribution meter is connected with one end of this looped optical fiber sensor to cause a pulse light to be incident on the looped optical fiber sensor and receive a Brillouin scattered light from the looped optical fiber sensor based on the pulse light, thereby measuring the distortion. In this structure monitor system, a change in the shape of the tunnel can be continuously monitored in a place remote from a spot, and three-dimensional distortions can be detected with high precision by the looped optical fiber sensor.
However, since the optical fiber sensor is laid on the inner wall surface of the tunnel in the above structure monitor system, distortions at positions near the inner wall surface (outer surface) of the tunnel can be detected with high precision, but it has been difficult to measure distortion inside the inner wall, particularly those at positions distant from the optical fiber sensor.
Further, the optical fiber sensor cannot be minutely laid on the inner wall surface of the tunnel for the technical or economical reason, and is laid along the circumferential direction of the tunnel at specified intervals in the longitudinal direction of the tunnel. Thus, it has been difficult to highly precisely measure distortions at positions where the optical fiber sensor is not laid even though these distortions are located at positions near the inner wall surface of the tunnel.
Accordingly, with the above prior art structure monitor system, it is difficult to highly precisely detect distortions at positions inside the inner wall of the tunnel and those at positions where the optical fiber sensor is not laid although these positions are located near the inner wall surface. For example, even if a crack is created inside the inner wall as the distortion increases, only rough monitoring can be carried out. It is necessary to go to the spot and carry out a minute reexamination in order to specify the position and size of the crack. This leads to an inconvenience of necessitating much time and labor.